This invention relates generally to pumps and more particular, but not exclusively to high specific speed pumps.
The invention is particularly applicable to high speed pumps typically used for flue gas desulphurisation (FGD) applications in power stations. Such pumps are utilised in a wet limestone scrubbing process to circulate the slurry to remove the sulphur from the flue gases before the sulphur can enter the atmosphere. Reference to this particular application is not to be taken as a limitation to the scope of the invention. It will be readily apparent to those persons skilled in the art that the invention is also applicable to other applications.
Pumps required for FGD applications typically must deliver high flowrate at moderate to low head. To achieve this they can, for example, be direct coupled which increases the speed at which they operate above, say, a lower speed gearbox driven pump. These higher speed pumps for a FGD application can also be referred to as a mixed flow pump, as opposed to, say, a normal slower running radial style of slurry pump. The flow in a radial pump is predominantly radial, whereas a mixed flow pump, the flow is both radial and axial.
The duty specifications for FGD pumps are normally very stringent and users require high efficiency. For FGD plants to operate correctly and efficiently, the FGD pumps must pump a precise set volume of limestone slurry through the FGD system. As the volume flowrate needs to be set precisely, so does the head (or pressure) that the pump supplies. Normally, the pump specification does not allow any negative tolerances, e.g., if the specified pump head is 25 m, then on test, the pump must produce 25 m or more. How much is also normally spelled out in the Pump Test Standard Acceptance Criteria that are given in the contract. This can be, say, +5% more head.
Design of a pump for FGD must take account of the duty requirements, particularly in regard to head as it is the head, which ultimately controls the volume flow slurry in the FGD system. With a direct driven mixed flow pump, the impeller diameter can only be changed marginally to meet the duty requirements. Hence there will be occasions that the pump would generate more head than specified and in some cases even more than the allowable upper limit specified in the contract. When the head is higher than the allowable tolerance, it must be reduced so the final tested head is within the tolerance band to meet the acceptance criteria.
To reduce the head to within the allowable tolerance (say xe2x88x920% to +5%), the impeller can be modified by causing it to be trimmed i.e. a. small reduction is made to the impeller diameter. Trimming to reduce the head also changes the power absorbed by the pump and this impacts on the pump efficiency.
The currently known technique as detailed in technical papers and texts is to make an angled cut on the drive side or rear shroud side of the impeller. This is shown in FIG. 1. Depending on the amount of trim (diameter reduction), the head and power are affected at different rates as shown in FIG. 2. Hence while the prediction of the necessary trim is complicated, the main problem relates to the fact that the head and power reductions do not follow the same pattern. Hence, trimming by this known method generally results in a reduction in head, a lesser reduction in power and a consequential decrease in efficiency. Hence it may occur that the head and flow produced by the pump are correct, but that the power absorbed is higher than the allowable tolerance. In such a case, the pump would be unacceptable.
It is an object according to one aspect of the present invention to provide a method of modifying a pump impeller so that it alleviates the problems discussed above.
It is a further object according to another aspect of the present invention to provide an improved pump impeller.
According to one aspect of the present invention there is provided a method of-modifying a pump impeller so as to achieve selected operating performance parameters, the impeller including a front shroud and a rear shroud, the shrouds being spaced apart so as to form a plurality of passageways therebetween which are separated by a plurality of impeller blades, each having an outer edge extending between the front and rear shrouds, the impeller having an outer diameter D, the method including the steps of trimming the outer edge of the impeller blades so that the outer diameter D1 of the front shroud is less than the outer diameter D2 of the rear shroud.
Preferably, the outer peripheral edge is trimmed so as to taper inwardly from the outer diameter D2 of the rear shroud to the outer diameter D1 of the front shroud.
According to another aspect of the present invention there is provided an impeller for a pump, the impeller including a front shroud having an outer diameter D1 and a rear shroud having an outer diameter D2, the shrouds being spaced apart so as to form a plurality of passageways therebetween which are separated by a plurality of impeller blades each having an outer edge extending between the front and rear shrouds charaterised in that the outer diameter D1 of the front shroud is less than the outer diameter D2 of the rear shroud.
Preferably, the ratio of D1/D2 ranges from between 1.0 to and including 0.85.
It has been surprisingly discovered that the reduction in head and power follows a more predictable pattern and more importantly, the reduction in head and power achieved for any trim are more equal. The effect is that the pump efficiency is far less affected by the trim as compared to the known method. Hence it is more likely using the new method of impeller trimming, that the head and flow can be achieved within tolerance and at the same time that the efficiency is likewise acceptable.